Power Transmission

ABSTRACT

An improved device and system for power transmission. A power transmission device comprises a primary winding magnetically coupled to a resonant secondary which comprises a plurality of magnetic resonators, each magnetic resonator comprising a magnetic winding. The magnetic resonators are connected in series and arranged so that the magnetic axis of each magnetic resonator is coupled to the primary winding. In operation, a power source supplies alternating current at an operating frequency to the primary of a power transmission device used as a transmitter. A load is coupled to the primary of a power transmission device used as a receiver. Collectors may be coupled to either or both of the transmit or receive device. A return line may be coupled to either or both of the transmit or receive device.

This document is a continuation of U.S. Utility patent application Ser.No. 13/718,238, filed on Dec. 18, 2012 on behalf of first-named inventorMichael Simon for “Improved Power Transmission,” which in turn claimspriority to U.S. Provisional Patent Application No. 61/631,633, filed onJan. 9, 2012 for “Pabellon effect wireless power transfer usingelectronically small resonant elements for near field tunneling.” Eachof these prior patent applications is incorporated herein by reference.

The present invention relates generally to electromagnetic powertransmission.

BACKGROUND

The goal of electric power transmission is the efficient transfer ofpower over distance. Improvements to power transmission look toimproving efficiency, distance, or both.

Electromagnetic power transmission transfers power between a powertransmitting device such as a transmit coil, and a power receivingdevice such as a receive coil or winding, through the use of inductivelycoupled magnetic fields. Power from an alternating current source isapplied to the transmit coil, creating a magnetic field. This magneticfield induces a magnetic field in the receive coil, generating analternating current in the second coil which is supplied to a load.

Magnetic power transfer according to the art may be characterized by thetype of coupling between the power transmitting device and the powerreceiving device. The three broad categories of such coupling are:transformer coupling, inductive coupling, and resonant inductivecoupling. An important aspect of each type of coupling is the distancebetween power transmitting and power receiving devices over which powertransfer is efficient

In transformer coupling, the transmit and receive coils are mountedclose together. In the case of transformers, transmit and receive coilsare commonly referred to as primary and secondary. At low frequencies,for example 20 to 20,000 Hertz (Hz) in audio use, transformers usemagnetic materials such as iron, steel, or ferrites for cores. Anexample of such a transformer would be the output transformer in avacuum tube guitar amplifier. In such a transformer, the primary andsecondary windings are placed on a laminated steel core, with onewinding wound on top of the other, thus providing tight magneticcoupling between primary and secondary windings. Air-core transformersare used for higher frequencies. An example of an air-core transformeris an intermediate-frequency (IF) transformer used in radio ortelevision equipment. Primary and secondary windings are woundmillimeters apart on a common nonmagnetic bobbin or form providing acommon axis, again providing tight magnetic coupling.

Inductive coupling may be thought of as a transformer with separateprimary and secondary windings which do not necessarily share a commoncore. Examples of inductive coupling include devices such asrechargeable electric toothbrushes and devices adapted to use chargingmats. In a rechargeable electric toothbrush, the transmit coil ismounted in a base unit into which the electric toothbrush body isinserted; the electric toothbrush body contains the receive coil whichrecovers power from the magnetic field produced by the transmit coil.Power from the receive coil in the form of alternating current isconverted to direct current to recharge a battery in the electrictoothbrush. In charging mats, such as the Duracell Powermat®, the matcontains the transmit coil which produces a varying magnetic field.Devices to be charged, such as phones or other handheld devices must beadapted for charging, such as by designing the device with a receivecoil and other circuitry for using the charging mat, or through addingan accessory such as a case containing the receive coil and chargingcircuitry which converts the alternating current from the receive coilto direct current for charging the device. The device to be charged mustbe placed directly on to the charging mat for charging to take place.For inductive coupling, the two coils must be close together, in themillimeter to centimeter range, for efficient power transfer.

In resonant coupling, the transmit coil is configured to resonate at achosen frequency, and alternating current is fed to the coil at thisfrequency. The transmit coil may be self-resonant, where the inductanceand self-capacitance of the coil set the resonant frequency, or the coilmay be made resonant by adding a capacitor in series or in parallel withthe coil. When driven at the resonant frequency, a coil is said to ring,generating an increasing oscillating magnetic field. If both transmitand receive coils are resonant, they must be carefully tuned to beresonant at the same frequency. Resonant inductive coupling can transferpower over what is considered the electromagnetic near field, defined interms of the wavelength (λ) of the operating resonant frequency, and inthe range of the wavelength divided by two Pi (λ/2π). Even in this nearfield, efficiency in resonant inductive coupling falls off at a rateproportional to one over the distance between transmitter and receiverto the fourth power.

To increase the efficiency of resonant inductive coupling, losses in thecoils are to be minimized. Common methods to reduce such losses includeusing air core coils to eliminate losses from magnetic cores, and usingphysically large coils with a small number of turns to reduce resistivelosses. This higher efficiency, measured electrically as the Qualityfactor or “Q” of a tuned circuit, results in a smaller bandwidth, oroperating frequency range; the higher the “Q”, the narrower thebandwidth. Such coils, when operating in the one to fifteen MHzfrequency range, may be up to a meter or more in diameter, provide powertransmission over a range of only a few meters, and only operate over avery narrow bandwidth.

In summary, transformer coupling is efficient but requires closelymounted coils, commonly with coils wound on a shared core. Inductivecoupling is efficient with separation of transmit and receive coils onthe order of millimeters to centimeters. Resonant inductive couplingextends the separation of transmit and receive coils to a meter or two,using physically large coils to reduce losses, and coils which have avery narrow bandwidth and are therefore operated at a fixed frequency.

What is needed is a way to increase the distance and efficiency inelectromagnetic power transmission.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of a power transmission device according to anembodiment.

FIG. 2 is a diagram of a power transmission device according to anotherembodiment.

FIG. 3 is a diagram of a power transmission device according to anotherembodiment.

FIG. 4 is a diagram of a power transmission system according to anembodiment.

FIG. 5 is a graph of power transmission performance according to anembodiment.

FIG. 6 is a diagram of a power transmission system according to anembodiment.

FIG. 7 is a diagram of a power transmission system according to anotherembodiment.

FIG. 8 is a graph of power transmission performance according to anembodiment.

FIG. 9 is a diagram of a power transmission system according to anembodiment.

DETAILED DESCRIPTION

Described herein are various embodiments of an improved powertransmission device comprising a primary winding magnetically coupled toa resonant secondary comprising a plurality of magnetic resonatorselectrically connected in series and arranged so that the magnetic axisof each magnetic resonator is in parallel with the magnetic axis of theprimary winding. Each power transmission device has a self-resonantfrequency.

In operation a power transmission device is used as a transmittingdevice, with an alternating current power source coupled to the primarywinding of the transmitting device. One or more power transmissiondevices are used as receiving devices, with the primary winding of eachreceiving device coupled to deliver power to a load.

In different embodiments, a capacitor may be placed in parallel with theresonant secondary.

In different embodiments, a collector may be coupled to either or bothof the transmitting and/or receiving devices.

In different embodiments, a return may be coupled to either or both ofthe transmitting and/or receiving devices.

Multiple receiving devices may be driven by a single transmittingdevice.

The operating frequency for the system comprising the transmittingdevice and one or more receiving devices is not necessarily theself-resonant frequency of the transmitting device or the receivingdevices. In one embodiment, the operating frequency is determined bysweeping a range of frequencies including the self-resonant frequenciesof the transmitting device and the receiving devices, selecting anoperating frequency which minimizes reflected power from thetransmitting device to the power source. This operating frequency maychange with the number of receivers present. Different operatingfrequencies may be used to preferentially provide power to one or morereceiving devices in a group. Multiple frequencies may be supplied to atransmitting device to support receiving devices operating on differentfrequencies.

Referring now to FIG. 1, an embodiment of a power transmission device100 in a top view is shown. Power transmission device 100 a comprises aprimary winding 110 a connected to primary terminals 112 and 114.Primary winding 110 a is magnetically coupled to a resonant secondary120 a. Resonant secondary 120 a comprises a plurality of magneticresonators 125 a. As shown, in this embodiment each magnetic resonator125 a has a rectangular cross section. Primary 110 a is wound aroundresonant secondary 120 a. While schematically only one turn is shown forprimary winding 110 a, primary winding 110 a typically comprisesmultiple turns.

Each magnetic resonator 125 comprises a winding; the plurality ofmagnetic resonators 125 are connected in series 128 a and connected tosecondary terminals 122 and 124. The plurality of magnetic resonators125 a are arranged so that the magnetic axis of each magnetic resonator125 a is in parallel with the magnetic axis of primary winding 110 a.

It is understood by those in the art that for primary winding 110 a tobe magnetically coupled to resonant secondary 120 a, primary winding 110a and each of the plurality of magnetic resonators 125 a comprisingresonant secondary 120 a are wound and connected in phase. Each magneticresonator 125 a and the primary winding 110 a are wound in helicalfashion; each may be a single layer winding, or a multiple layerwinding, one layer wound on top of the preceding layer. Othertraditional winding techniques known to the solenoid and inductor artsmay also be used. Magnetic resonators 125 a are arranged, as shown, toenclose a central area 130. A split-ring configuration may be used insome embodiments, where a gap 140 exists between first and last magneticresonators 125 b connected in series.

A useful approximation is that the total length of the winding wire usedto form the plurality of magnetic resonators 125 comprising resonantsecondary 120 is one half the wavelength of the resonant frequency.Recall that the wavelength, usually shown as the Greek letter lambda(λ), is equal to the velocity of the speed of light divided by thefrequency. As an example, for a frequency of 2 MHz, the wavelength isapproximately 150 meters; half this wavelength is 75 meters, or 491feet. This 75 meters of wire would be distributed over the number ofindividual magnetic resonators 125. As an example, in an embodimentusing nine magnetic resonators 125, each magnetic resonator 125 woulduse approximately eight and one third meters of wire.

FIG. 2 shows an additional embodiment of power transmission device 100,again in top view. In this embodiment, magnetic resonators 125 b have acircular cross section. As in FIG. 1, magnetic resonators 125 b mayenclose a central area 130 b. A split-ring arrangement may be used, witha gap 140 b between first and last magnetic resonators 125 b connectedin series.

In one embodiment, primary 110 b is wound around the plurality ofmagnetic resonators 125 b, for example, ten turns around the outercircumference of the plurality of magnetic resonators 125 b.

In another embodiment, primary 110 b is wound around each magneticresonator 125 b in series, for example, ten turns wound around a firstmagnetic resonator 125 b, continuing to ten turns wound around a nextmagnetic resonator 125 b, and so on for each of the plurality ofmagnetic resonators 125 b.

FIG. 3 shows an additional embodiment of power transmission device 100suitable for implementation in a planar form, for example using printedcircuit board fabrication techniques. As shown, magnetic resonator 125 cis formed as a conductive trace for example on one side of a printedcircuit board. While a rectangular trace is shown, a spiral trace mayalso be used. Connections 128 c may be made for example using a wireextending out of the substrate containing the traces forming magneticresonator 125 c, or in the case of a multi-layer substrate, such as adouble-sided printed circuit board, connections 128 c are made using aconductive trace on the other side of the substrate from the conductivetraces forming magnetic resonator 125 c, connecting for example usingplated-through vias as is known to the printed circuit board arts.

Primary winding 110 c is similarly formed as a conductive trace on asubstrate, such as the same substrate used for magnetic resonators 125c.

In some embodiments of FIG. 3, fabrication techniques used inmulti-layer printed circuit boards may be used to extend magneticresonators 125 c and/or primary winding 110 c through multiple layers ofa multi-layer printed circuit board by stacking multiple traces on topof each other, interconnected using vias and traces on other layers.

In some embodiments, a collector is coupled to either primary 110 orresonant secondary 120 of a power transmission device 100. In practice acollector is a generally flat conductive surface, such as a rectangular,square, or circular piece of conductive material such as a metal foil.In one embodiment, aluminum foil and Kevlar® backed aluminum foil havebeen used. In another embodiment, surfaces treated with a conductivecoating, such as the spray-on EMI/RFI shielding carbon conductivecoating, Catalog No. 838 from MG Chemicals may be used. Collector sizeis roughly inversely proportional to frequency, with larger collectorsbeing used at low frequencies. Collector size varies from a few squareinches to a few square feet. The collector is electrically coupled tothe device by a wire. As explained in more detail elsewhere herein, aninductor may be used to couple the device and the collector. A singlecollector may be connected to multiple power receiving devices.

A collector connected to a power transmission device acts to broaden thefrequency response of the device. The frequency response of the devicemay also be changed, for example, by placing a resistor across thesecondary, that is, by connecting a resistor between the secondaryterminals 122 and 124, which also broadens the response. Placing acapacitor across the secondary narrows the response, turning thesecondary into a tuned circuit. Other resonating elements may also beused to shape the frequency response of the secondary, and by shapingthe frequency of the secondary, shaping the frequency response of theoverall power transmission device.

It is believed that the collector alters the permeability of the system.By tuning the size of the collector, and optionally adding an inductorin series with the collector and tuning the value of the inductor, thesystem permeability may be made to approach zero. Collector size andinductor value may be tuned by observing power transfer in a system andadjusting collector size and/or inductor value to increase powertransfer, measured as power available at one or more receiving devicesin the system.

Similarly, in some embodiments an electrical link forming a returncouples the power transmitting device to one or more power receivingdevices. The return may be a direct electrical link such as a wirebetween transmitting and receiving devices. A return may also beprovided, for example, by the standard electrical power wiring in afacility, which provides a ground connection at each electrical outlet.An inductor may be placed between the device and the return. It isbelieved that the return alters the permittivity of the system, loweringthe permittivity. An inductor in series with the return allows thepermittivity to be further tuned, approaching zero. The value of theinductor may be selected by observing power transfer in a system, andadjusting inductor value to increase power transfer, measured as poweravailable at one or more receiving devices in the system.

In one embodiment, two power transmission devices according to FIG. 2were constructed in the same manner and labeled A and B. Resonantsecondary 120 consists of six magnetic resonators 125. Each magneticresonator consists of 13 turns of solid 30 gauge (30 AWG) Kynar®insulated wire in a single layer on a 0.5 inch diameter wood dowel form.The six resonators were spaced equally around a 1.25 inch diametercircle and connected in series. The magnetic resonator windings werewrapped with Kapton® tape. Primary 110 consists of six turns of solid 30gauge Kynar® insulated wire wound on top of each magnetic resonator 125in series.

Self-resonant frequencies for each device were determined using acalibrated HP 8594E Spectrum Analyzer with HP 85630A Scalar Test Set andHP 85714A Scalar Measurements Personality, producing the following data:

TABLE 1 Scalar Measurements Device Resonance Return Loss A 75.4 MHz   −3dBm 67.5 MHz −1.1 dBm 46.2 MHz   −1 dBm 39.5 MHz −1.8 dBm B 74.6 MHz−2.8 dBm 65.4 MHz −0.8 dBm 45.4 MHz −0.8 dBm 38.8 MHz −1.6 dBm

The return loss is a measure of the impedance mismatch between a sourcedevice and a terminating load. A more negative value for return lossindicates a better impedance match between the source, in this case theScalar Analyzer, and the terminating load, in this case the powertransmission device. Return loss may also be thought of as a measure ofhow much power from the source is reflected back from the load andtherefore unused. The more negative the return loss, the less power isbeing reflected from the load, and therefore more power is going to theload.

Referring to FIG. 4, power transfer over distance was measured, usingdevice A as power transmitting device 410, and device B as powerreceiving device 430. Measurements were first made without a return 450,and then with a return 450 electrically connecting the primary 110 ofpower transmitting device 410 to the primary 110 of power receivingdevice 420. The power source 400 used for these measurements was thetracking generator of the HP 8594E Spectrum Analyzer coupled through theHP 5630A Scalar Test Set, which produces a sine wave. Source power totransmitting device 410 was −10 dBm (0.10 milliWatts, or mW).

Power measurements without return 450 were made using a calibratedprototype battery-operated power meter as load 430, allowing powerreceiving device 420 and the power meter connected as load 430 to befloating, unconnected to other equipment. Power measurements with return450 were made using the HP 8594E Spectrum Analyzer coupled through theHP 5630A Scalar Test Set, with the ground connection of the coaxialcables used to connect the Test Set to power transmitting device 410 andpower receiving device 420, providing the return path from primary 110of power transmitting device 410 to the primary 110 of power receivingdevice 420.

An operating frequency of 44.61 MHz was selected for the test by placingpower transmitting device 410 and power receiving device 430 close toeach other (approximately four inches separation), both devicesconnected to the Scalar Test Set, and selecting the frequency with thehighest peak as displayed on the Spectrum Analyzer. Distance 440 wasmeasured between the center of power transmitting device 410 and powerreceiving device 420. Power measurements were made as follows.

TABLE 2 Power Transfer without Return and with Return Distance withoutwith Inches Return Return 2   −38 dBm −32.4 dBm 3 −40.3 dBm 4 −43.1 dBm  −33 dBm 5 −45.1 dBm 6 −48.1 dBm 7   −51 dBm 8   −53 dBm −33.6 dBm 9  −55 dBm 10   −56 dBm 11 −58.4 dBm 12 −60.2 dBm 16   −65 dBm −34.5 dBm18 −66.8 dBm 32 −36.3 dBm 40   −37 dBm 64 −38.1 dBm 80 −35.7 dBm 128−35.4 dBm 160 −38.5 dBm 276 −39.9 dBm

This data is also shown in graphical form in FIG. 5. This data and theassociated graph show the rapid falloff of power expected with resonantinductive coupling for the no return configuration.

Note that the decibel (dB) scale is a logarithmic scale; a change ofapproximately 3 dB represents a doubling of power. A change of 20 dBrepresents a hundred times increase in power. Similarly a change of −3dB represents a halving of power, and a change of −20 dB represents onehundredth the power. The dBm scale uses a reference level (0 dB) of 1milliWatt (mW) referenced to a 50 Ohm load.

In contrast, with an electrical link forming return 450 connecting theprimary 110 of power transmitting device 410 and the primary 110 ofpower receiving device 420, power transfer is approximately level withdistance.

It should be noted that the wavelength for a frequency of 44.61 MHz is854 inches. The traditional limit of the near-field range for standardresonant inductive coupling is considered to be the wavelength (λ)divided by two Pi (λ/2π). For the frequency used, 44.61 MHz, thisnear-field limit is 136 inches, as shown on the graph of FIG. 5. Thedata in Table 2 as shown in the graph of FIG. 5 for a configuration witha return show approximately linear power transfer continuing past thenear field limit, and measured at past twice this near field limit.

In another embodiment, four power transmission devices according to FIG.1 were constructed. These devices were labeled A, B, C, and D. Theoverall dimensions of each of the four devices are 4 inches in width by4 inches in length by 1.375 inches in height (101 mm×101 mm×35 mm).Primary winding 110 consists of 18 turns of 16 gauge insulated magnetwire. Resonant secondary 120 comprises nine magnetic resonators 125,each magnetic resonator 125 with dimensions of 1.375 inches×0.5inches×1.375 inches in height (35 mm×12.7 mm×35 mm). Each magneticresonator 125 comprises 52 turns of 16 gauge insulated magnet wire woundin three layers. The nine magnetic resonators 125 are connected inseries, with layers of Kapton® tape providing insulation betweenresonant secondary 120 and primary winding 110.

Each of these devices has a resonant frequency of approximately 2 MHz.This self-resonance arises from the combined length and inductance ofthe windings of magnetic resonators 125 and the distributed capacitancein these windings. It should be noted that these devices are physicallysmall in relation to the frequency (2 MHz) and wavelength (150 meters).

Power transmission devices A, B, C, and D were tested to determine theself-resonant frequency and return loss of each device at itsself-resonant frequency, with and without a collector attached toterminal 122 of resonant secondary 120. These tests were made using theHP 8549E Spectrum Analyzer with HP 85630A Scalar Test Set and HP 85714AScalar Measurements Personality.

TABLE 3 Scalar Measurements Resonant Return Device Frequency Loss A nocollector 2.180 MHz  −9.7 dB A with collector 2.075 MHz −13.0 dB B nocollector 2.213 MHz −6.42 dB B with collector 2.083 MHz −8.83 dB C nocollector 2.163 MHz −8.09 dB C with collector 2.068 MHz −11.23 dB  D nocollector 2.133 MHz  −7.8 dB D with collector 2.043 MHz −10.66 dB 

The data of Table 3 show that adding a collector to a device lowers itsself-resonant frequency and decreases (improves) the return loss. Inthese tests, the collector was a square piece of Kevlar®-backed aluminumfoil approximately eight inches on a side, connected to the resonantsecondary 120 of the device under test.

FIGS. 6 and 7 show test configurations using these devices to measurepower transfer over distance. FIG. 6 shows the test setup for measuringpower transfer over distance between two power transfer devices, withoutthe use of collectors. FIG. 7 shows the test setup for measuring powertransfer between power devices using a collector and a return. Theresults of these tests are shown in the graph of FIG. 8.

The data for the “without return” line of FIG. 8 is generated by thetest configuration shown in FIG. 6. Device A previously described isused as power transmitting device 610 and device C is used as powerreceiving device 620. In testing, power source 600 consists of an HP3314A function generator producing a triangle wave driving a Verteq VPA1987 Power Amplifier. Load 630 was an HP 8594E Spectrum analyzer, usedfor making measurements, connected to the primary 110 of receivingdevice 620. The Spectrum Analyzer provides a 50 Ohm load on the primary110 of receiving device 620. Forward power from the amplifier totransmitting device 610 was approximately 50 watts, with return powerapproximately 8 watts. The operating frequency was 2.093 MHz, chosen toprovide maximum measured output at receiving device 620. This operatingfrequency is different from the self-resonant frequencies of bothdevices. Measured power in dBm for various distances 640 betweentransmitting device 610 and receiving device 620 are shown in Table 4.

TABLE 4 Receive Power in dBm as a Function of Distance (without return)Dist Power inches meters dBm 1 0.0254 7.5 3 0.0762 3.7 7 0.1778 −5.3 120.3048 −15 27.5 0.6985 −34 54 1.3716 −50

The measurements shown in Table 4 are in agreement with the rapidfalloff of power associated with resonant inductive coupling.

FIG. 7 is a diagram of a power transmission system according to anembodiment. In this embodiment, transmitting device 710 has a collector715 connected to terminal 122 of resonant secondary 120 of transmittingdevice 710. Power source 700 is the HP 3314A Function Generator andVerteq VPA 1987 Power Amplifier as used in the preceding test.

Multiple receiving devices 720 and 740 are used. Receiving device 720has a high efficiency red light emitting diode (LED) connected to itsprimary as load 730. Receiving device 740 is connected to the HP 8594ESpectrum Analyzer as load 750 for measurements, providing a 50 Ohm loadto the primary 110 of receiving device 740. As shown resonantsecondaries 120 of receiving devices 720 and 740 are connected togetherand coupled through a 220 microHenry (μH) inductor 770 to the casegrounds of the HP 5514A signal generator and the Verteq VPA 1987 PowerAmplifier, and to the primary 110 of transmitting device 710, thusproviding a return. The other terminal of the resonant secondaries fortransmitting device 710 and receiving devices 720 and 740 areunconnected.

An operating frequency of 2.093 MHz was selected as providing best powertransfer from transmitting device 710 to receiving devices 740 and load750. During measurements, the LED used as load 730 for receiving device720 was illuminated by power received from transmitting device 710.

Receive power as a function of distance 760 between collector 715 ontransmitting device 710 and receiving devices 720 and 740 in thisconfiguration is shown in Table 5.

TABLE 5 Receive Power in dBm as a Function of Distance (with return)Dist Power inches meters dBm 13 0.3302 11 36 0.9144 4.05 58 1.4732 3.182 2.0828 2.76 96 2.4384 3.4 106 2.6924 3.5 118 2.9972 3.5 130 3.302 4142 3.36068 3.68 154 3.9116 3.4 168 4.2672 3.84

The measurements in Table 5 show constant power transfer scaling withdistance, shown as the “with return” line of the graph of FIG. 8. Thisis in stark comparison to the rapidly nonlinearly diminishing powertransfer shown in Table 4, and the “without return” line of the graph ofFIG. 8.

The embodiment of FIG. 7 shows one transmitting device providing powerto two receiving devices simultaneously. This embodiment shows atransmitting device with a collector coupled to the secondary. Thisembodiment shows two receiving devices coupled to a common returnthrough an inductor.

Additional embodiments show this constant power scaling with range suchas shown in Table 4.

In another embodiment, a pair of power transmission devices according toFIG. 1 were constructed. The resonant secondary 120 of each devicecomprises 17 magnetic resonators 125. Each magnetic resonator 125 is 44turns of 16 AWG magnet wire wound in two layers on a form measuring 1.5inches wide by 0.75 inches long by 1.5 inches in height. These magneticresonators 125 are arranged in a generally square form with a gapbetween first and last magnetic resonators. The resonant secondary wascovered with Kapton® tape to provide insulation between primary andsecondary, and wound with the primary 110, 16 turns of 16 AWG magnetwire. The overall size of each device is 8 inches long by 8 inches deepand 1.5 inches high.

Self-resonant frequencies and return loss were determined for eachdevice. The transmitting device showed a self-resonant frequency of 1.63MHz with a return loss of −3.95 dB without a collector, and 1.51 MHzwith a return loss of −2.75 dB with a collector. The receiving devicemeasured a self-resonant frequency of 1.34 MHz with a return loss of−6.2 dB without a collector, and 1.27 MHz with a return loss of −7.11 dBwith a collector

Power transfer was measured using the embodiment of FIG. 9. Power source900 was the HP 5514A signal generator and Verteq VPA 1987 PowerAmplifier, coupled to primary 110 of transmitting device 910. Measureddrive power was 27.5 dBm at 1.490 MHz, as measured using the portable RFpower meter connected to the Power Amplifier output through a precision20 dB power attenuator. A collector 915 was connected to resonantsecondary 120 of transmitting device 910. Collector 915 was a circularcardboard disc approximately one foot in diameter, coated with aspray-on EMI/RFI shielding carbon conductive coating as describedpreviously herein.

Primary 110 of receiving device 920 was coupled to the portable RF powermeter as load 930. A return 940 connected from the primary 110 oftransmitting device 910 to the primary 110 of receiving device 920comprising a length of 30 AWG solid Kynar® insulated wire was also used.A collector 926 was also coupled to the secondary 120 of receivingdevice 920 through a 100 microhenry inductor 926. Collector 926 was asmall aluminum sheet metal plate approximately 8 inches by 16 inches.

During testing, a metal ring 912 consisting of a piece of 1 1/2 inchwide adhesive-backed aluminum tape was placed inside resonant secondary120 of transmitting device 910. In the configuration of FIG. 9 withoutcollectors 915 and 926 in place, a power level of 5.6 dBm was measuredat receiving device 920 without ring 912 present. Placing ring 912 intotransmitting device 910 increased the power measured at receiving device920 to 7.7 dBm, an increase of 1.1 dBm.

Distance testing was performed outdoors on an asphalt surface. With apower input of 27.5 dBm at 1.545 MHz to transmitting device 910, thepower levels measured at receiving device 920 over distance are asshown.

TABLE 6 Receive Power over Distance (with return and collectors)Distance Receive Power Feet dBm 4 13.7 dBm 11 12.8 dBm 32.5 13.0 dBm 8113.4 dBm

This shows the constant power transfer scaling with distance consistentwith other embodiments.

The effect of inductor 924 coupling collector 926 to the secondary 120of receiving device 920 was readily apparent in this test. With inductor924 in place coupling collector 926 to the secondary 120 of receivingdevice 920, measured power was 13.4 dBm at a distance of 81 feet.Removing inductor 924 and connecting collector 926 directly to thesecondary 120 of receiving device 920 reduces the receive power at thesame distance to 8.6 dBm, a loss of 3.8 dB.

Testing also demonstrated that the collector may be coupled to theresonant secondary at other than terminals 122 and 124, the ends of theresonant secondary. For example, the collector may be coupled to theresonant secondary at the connection between a pair of magneticresonators 125.

Note that in the testing described herein, no attempt was made to matchthe output impedance of the power sources used, 50 Ohms in each case, tothe impedance of the transmitting device at the operating frequencychosen. Proper impedance matching is expected to increase efficiency andoverall power transfer. As examples, a return loss of −3 dB correspondsto a power loss of 50% between the power source and the transmittingdevice. A return loss of −6 dB corresponds to a power loss of 25%between the power source and the transmitting device. Similarly, noimpedance matching was done between the primary of the receiving deviceand the load on the receiving device. Load devices such as the SpectrumAnalyzer and portable RF power meter present a 50 Ohm load to thereceiving device. Loads such as incandescent lamps and light emittingdiodes (LEDs) present more complex loads, particularly in the case ofLEDs. Proper impedance matching should improve efficiency and overallpower transfer.

Different waveforms may be used to drive the transmitting device. Thetests described herein were performed using a combination of sine waves,square waves, and triangle waves as drive signals. Recall that a squarewave is the sum of a fundamental frequency and the odd harmonics of thefundamental; a triangle wave is also the sum of a fundamental frequencyand odd harmonics, but with a faster rolloff than in the case of asquare wave.

A combination or sum of drive signals may also be fed to a transmittingdevice. As an example, a first signal generator generating a drivesignal at 1.9 MHz, and a second signal generator generates a drivesignal at 2.1 MHz. These two drive signals are added and fed to theinput of the power amplifier, which is coupled to the primary of thetransmitting device. Each drive signal powers a group of receivingdevices.

It has been learned that when a transmitting device and one or morereceiving devices are operated together, they operate as a system. Theoperating frequency of this system is not necessarily the self-resonantfrequency of any of the individual devices. In one embodiment, anoperating frequency is determined by sweeping a range of frequenciesincluding the self-resonant frequencies of the devices in the systemwhile measuring forward and reverse power from the power source to thetransmitting device. An operating frequency will have a low ratio ofreverse to forward power. There may be more than one such frequencywithin a frequency range. As an example, with a transmitting devicehaving a self-resonant frequency of 1.6 MHz, and a group of receivingdevices having self-resonant frequencies in the neighborhood of 2 MHz,sweeping from 2.1 MHz down to 1.5 MHz while monitoring the ratio ofreverse to forward power will identify one or more operatingfrequencies. In another embodiment, the frequency sweep is made at a lowpower level, with power increased once an operating frequency has beenselected.

When loads presented by individual receiving devices change, or morereceiving devices and loads are added to the system, the overall systemresonance changes. This change alters the ratio of reverse to forwardpower measured at the transmitting device, and also changes the powerlevel present at other receiving devices. One embodiment uses thisdetected change at the transmitting device to initiate a search for apossibly better operating frequency. In another embodiment, receivingdevices use shifts in load to signal the transmitting device and/orother receiving devices, for example toggling a load to send serial datato another device.

We hypothesize that the transmitting and receiving device embodimentsdescribed herein implement a metamaterial lens with effectivepermittivity and/or permeability approaching zero or equal to zero. Thishypothesis is derived using transformation optics techniques. Thederivation considers two spaces, physical space and electromagneticspace. Physical space is the space within which the devices arephysically present. Electromagnetic space is the space perceived by theelectromagnetic energy present in the system. The derivation shows thatpoints separated in physical space are made approximately coincident inelectromagnetic space by using devices with effective permittivity andpermeability approaching zero or equal to zero. This coincidence inelectromagnetic space allows for efficient power transfer. The design ofthe devices described herein was informed by this theory.

It is to be understood that the examples given are for illustrativepurposes only and may be extended to other implementations andembodiments with different conventions and techniques. While a number ofembodiments are described, there is no intent to limit the disclosure tothe embodiment(s) disclosed herein. On the contrary, the intent is tocover all alternatives, modifications, and equivalents apparent to thosefamiliar with the art.

In the foregoing specification, the invention is described withreference to specific embodiments thereof, but those skilled in the artwill recognize that the invention is not limited thereto. Variousfeatures and aspects of the herein-described invention may be usedindividually or jointly. Further, the invention can be utilized in anynumber of environments and applications beyond those described hereinwithout departing from the broader spirit and scope of thespecification. The specification and drawings are, accordingly, to beregarded as illustrative rather than restrictive. It will be recognizedthat the terms “comprising,” “including,” and “having,” as used herein,are specifically intended to be read as open-ended terms of art.

We claim:
 1. A device for power transmission comprising: a primarycomprising a conductive winding; and a resonant secondary having aresonant frequency, the resonant secondary magnetically coupled to theprimary, the resonant secondary comprising a plurality of magneticresonators, each magnetic resonator comprising a conductive winding andhaving a magnetic axis, the plurality of magnetic resonators connectedin series, with the magnetic axis of each of the plurality of magneticresonators magnetically coupled to the primary.